A TBI can increase the risk of dementia by 80 percent, even 15 years after an accident.

A concussion or other traumatic brain injury (TBI) can increase the risk of developing dementia even 30 years later, according to a new study published today.

Severe blows to the brain have been associated with dementia for some time, but the new study finds that the risk remains high years later.

Researchers from Umeå University in Sweden were able to look at a large trove of data via nationwide databases from 1964 to 2012.

They broke down the data into three cohorts to study. One examined 164,334 people who had a TBI, and were compared to an equally sized control group. Another looked at 136,233 individuals who had dementia diagnoses, and were matched with a control group.

The third looked at 46,970 pairs of siblings, where one had received a TBI diagnosis.

By looking at this large amount of data over decades, the researchers were able to find a clear association between TBIs and the risk of dementia, according to the study published today in PLOS Medicine journal.

Researchers found the risk of a dementia diagnosis was highest during the first year after the injury. During this time, people who had a TBI were 4 to 6 times as likely to get a dementia diagnosis as those without a TBI.

While the overall risk decreased over time, TBI patients still faced higher risk than those without a TBI, even 30 years later.

At the 15-year follow-up, the researchers found the risk for a dementia diagnosis increased 80 percent for people who had at least one TBI compared to those who didn’t have a TBI.

Dr. Steven Flanagan, chair of the department of rehabilitation medicine at the New York University Langone Medical Center, said this study adds to a body of research linking brain injury and dementia risk.

“The preponderance of the literature I would say supports that there appears to be an increased risk of dementia after traumatic brain injury,” he said.

Flanagan said the use of the sibling cohort could help experts better understand dementia risk and genetics, since the siblings with a TBI were more likely to develop dementia than their brother or sister.

“Not a great surprise, but it adds to the body of literature that has already supported this,” Flanagan said of the study results. “It brings us one step closer to understanding what this all means.”

The study doesn’t definitively say that a TBI causes dementia, just that the two appear to be linked.

Dr. Alan Lerner, director of the University Health Medical Center’s Brain Health and Memory Center, said that the study also showed that not all TBIs are the same.

“Trauma has a dose effect, and that’s something that people don’t really understand,” he said. “The people with mild TBI… as opposed to severe TBI, those people had less frequent dementia than the people with severe or repetitive [injury.]”

Lerner pointed out that “it’s worse if you fall off your bicycle a hundred times than if you fall off twice.”

However, he said this study spotlights the need to “prevent the next hit” so that dementia risk isn’t compounded by additional injury.

Flanagan and other experts say that an increased risk from a TBI doesn’t mean a person is doomed to develop dementia.

“What I tell my patients who are worried about this is not everyone with TBI gets dementia,” Flanagan said. Just as “not everyone who smokes gets emphysema and lung cancer.”

Dr. Gayatri Devi, a neurologist at Lenox Hill Hospital in New York, said that the study can help experts better monitor and help those at high risk for developing dementia.

“If there’s a way that we could perhaps track patients over time so we could determine whether they’re developing symptoms, [or] whether they’re developing signs even before they get their symptoms,” said Devi, who is also the author of “The Spectrum of Hope: An Optimistic and New Approach to Alzheimer’s Disease and other Dementias.”

Devi said that while there’s no medication to prevent the onset of dementia, there are lifestyle risk factors that can impact a person’s risk.

“If there’s a way we can follow them over time so we can intervene earlier before they began to develop symptoms,” Devi said, “that would be one way that we can use these risk-factor type studies to help reduce long-term risk for dementia.”

For example, people can reduce their risk of certain dementias by keeping a healthy weight and keeping blood pressure low.

“Lifestyle modification — it can reduce risk for something like Alzheimer’s between 30 to 50 percent,” said Devi.